The development of fairly consistent, accurate means to measure biological age - as opposed to chronological age - from a tissue sample is an important thread in aging research. Aging is a process of damage accumulation, and rejuvenation would be achieved through damage repair. Research and development aimed at significant extension of healthy life span can only become cost-effective given good ways to measure damage, however. There must be some reliable means to quickly assess the results of a treatment that claims a degree of rejuvenation through the partial repair of a specific form of cellular or molecular damage. In some cases this might seem easy. Take senescent cell clearance, for example: you run the therapy in mice, and compare a range of measures known to scale by senescent cell count in tissue samples before and after the treatment regimen. However, all that really tells you is how well the therapy clears senescent cells. All aspects of biology interact with one another, and age is a global phenomenon. To determine how aged an individual is and how effective a treatment might be when it comes to the practical outcome of additional healthy life span added there is presently little to be done other than wait and see.
The biggest challenge in the development of life-extending therapies is funding and cost. On the one hand there is far too little funding directed towards finding ways to treat aging. On the other hand effectively evaluating an alleged means of treating aging currently requires life span studies, and even in mice that takes far too long and costs far too much to be done casually. If there were standardized, quick and easy markers of physiological age that could be assessed before and after a treatment, then this research and development might be able to proceed ten times as rapidly, and evaluation of possible therapies would be open to far more research groups. There are many, many more laboratories with the capacity and funding to carry out a speculative $100,000 study versus a speculative $1,000,000 study.
All of this is to explain why there is considerable interest in developing a cheap biomarker of aging that can reliably assess physiological age from a tissue sample. No-one wants to run a five year mouse study if there is a ten minute alternative that produces an answer of about the same accuracy. That ten minute alternative doesn't yet exist, but some lines of research seem promising, such as work on DNA methylation patterns that appear to be fairly consistent between individuals over the course of aging. There is also the suggestion that the approach should be to measure the fundamental forms of damage thought to cause aging - but all of them, not just the one being treated by the therapy under consideration. At the present time that might be more onerous than finding a good set of secondary consequences that are reactions to damage, such as epigenetic changes.
The open access paper linked below covers a fairly wide range of topics. The structures of our cells and tissues are built of proteins, and these proteins are constantly damaged and replaced. Many varied mechanisms toil constantly to remove proteins and cellular components as soon as they show damage or dysfunction. Nonetheless the difference between young tissue and old tissue is that old tissues have far more damage: misfolded proteins, malfunctioning structures inside cells, metabolic waste products such as advanced glycation endproducts (AGEs) gumming together structures in between cells, and on and so forth. The damage leaks through, and even damage repair mechanisms are not invulnerable; they falter with age due to much the same set of issues as causes dysfunction elsewhere. In the future repair technologies, such as those outlined in the SENS proposals, will bring about rejuvenation by reversing these forms of damage. Since these issues are a part of full set of causes of aging they are also potential markers of aging.
Changes in the abundance and post-translational modification of proteins and accumulation of some modified proteins have been proposed to represent hallmarks of biological ageing. Non-enzymatic protein glycation is a common post-translational modification of proteins in vivo, resulting from reactions between glucose or its metabolites and amino groups on proteins, this process is termed "Maillard reaction" and leads to the formation of advanced glycation endproducts (AGEs). During normal ageing, there is accumulation of AGEs of long-lived proteins such as collagens and several cartilage proteins. AGEs, either directly or through interactions with their receptors, are involved in the pathophysiology of numerous age-related diseases, such as cardiovascular and renal diseases and neurodegeneration.
Beside protein glycation, it is also well known that levels of oxidised proteins increase with age, due to increased protein damage induced by reactive oxygen species (ROS), decreased elimination of oxidized protein (i.e. repair and degradation), or a combination of both. Since the proteasome is in charge of both general protein turnover and removal of oxidized protein, its fate during ageing has received considerable attention, and evidence has been provided for impairment of the proteasome function with age in different cellular systems. Thus, these protein maintenance systems may also be viewed as potential biomarkers of ageing.
It is expected that a combination of several biomarkers will provide a much better tool to measure biological age than any single biomarker in isolation. For the most part, the markers based on proteins and their modifications that have been chosen are directly related with mechanistic aspects of the ageing process. Indeed, they are relevant to such important physiological features such as protein homeostasis and glycoprotein secretion that have been previously documented as being altered with age. Therefore, it is expected that they may be less influenced by other factors not necessarily related with ageing.